Federal law requires you to maintain copies of your tax returns and supporting documents for three years. This is called the "three-year law" and leads many people to believe they're safe provided they retain their documents for this period of time. However, if the IRS believes you have significantly underreported your income (by 25 percent or more), it may go back six years in an audit. If there is any indication of fraud, or you do not file a return, no period of limitation exists.
Contrary to popular belief, there’s nothing in the U.S. Constitution or federal law that prohibits multiple states from collecting tax on the same income. Although many states provide tax credits to prevent double taxation, those credits are sometimes unavailable.
Generally, if you’re “domiciled” in a state, you’re subject to that state’s income tax on your worldwide income. Your domicile isn’t necessarily where you spend most of your time. Rather, it’s the location of your “true, fixed, permanent home” or the place “to which you intend to return whenever absent.” Your domicile doesn’t change — even if you spend little or no time there — until you establish domicile elsewhere.
Residence, on the other hand, is based on the amount of time you spend in a state. You’re a resident if you have a “permanent place of abode” in a state and spend a minimum amount of time there — for example, at least 183 days per year. Many states impose their income taxes on residents’ worldwide income even if they’re domiciled in another state.